Family knew from the start of diagnosis (October 2012 with Type 1). Managers and certain colleagues at work know because I was hospitalised with DKA coma at the time of diagnosis and off work for four weeks. I am 57 and don't have a problem telling people socially or at work, but it's now on a 'need to know' basis. Working in an office means lots of cakes and biscuits, so it's better to have an explanation rather simply refusing to eat them (I don't like cakes and not keen on biscuits!).
Going on a group holiday soon. The organisers know about the diabeties for insurance purposes, and I guess the other participants will find out when I start testing and injecting at meals. But I wouldn't go out of my way to tell them first.
When I've told colleagues and friends, they tell me that they have friends or relatives with late (adult) onset of Type 1, so that's been helpful. On the whole, I've found it better to tell people.